1. FIELD OF THE INVENTION
This invention relates to a superconductive oxide substance which can be used as the wire material for superconductive coil to generate high magnetic field and so forth, and also to a method for producing powder of such superconductive oxide substance.
2. DISCUSSION OF BACKGROUND
In recent years, there has been high demand for the superconductive coils, magnetic shielding materials, high frequency cavity resonators, etc. of low operating cost in the field of magnetically levitated train, high energy physics, nuclear magnetic resosnance apparatus, material property researches, and so on. Since the conventional superconductive coil has been manufactured in such a way that a superconductive wire made of an alloy having a low critical temperature, such as, for example, NbTi (having a critical temperature Tc of approximately 10 K.) or intermetallic compounds such as, for example, Nb.sub.3 Sn (having Tc of approx. 18 K.) is wound on a coil winding frame, the superconductive coil as produced had to be operated at a temperature of liquefied helium (4.2 K.). On account of this, high cost would inevitably incur to the cooling operation, which, in turn, redered the operating system complicated. In 1987, however, a superconductive oxide substances having a very high critical temperature of 100 K. or above were discovered, which made it possible to operate the superconductive coil at a temperature of liquefied nitrogen (77 K.). This superconductive substance is an oxide of a composition represented by (Ba.sub.0.6 Y.sub.0.4).sub.2 CuO.sub.4. The superconductive oxide is produced by mixing BaCO.sub.3, Y.sub.2 O.sub.3, and CuO, all being in powder form, shaping the mixture in a desired shape, and thereafter heat-treating the shaped article. The technique in this field is introduced in a periodical: Phys. Rev. 58 (1987), pp 908 to 910.
On the one hand, the superconductive wire to be used for the conventional superconductive coil is required to be a long continuous line, and to have a stable and excellent superconductive property in the longitudinal direction. However, since the superconductive oxide substance having a high critical temperature (Tc) belongs to ceramic which is very brittle, it is difficult to manufacture a long continuous wire with the conventional technique. On account of this, the effective way of producing the wire material for the superconductive coil is to deposit such superconductive substance on a long tape or fiber as the substrate by means of the chemical vapor deposition method (hereinafter abbreviated as "CVD"). However, due to absence of the materials having appropriate vapor pressure required to carry out the CVD, the desired CVD method could not be effected. Accordingly, the problem still remains in obtaining the superconductive oxide coil having a long length.
On the other hand, the superconductive wire to be used for the superconductive coil, for example, is required to have a high critical current density (Jc). For this purpose, the powder material for sintering is required to have accurate chemical composition, small particle diameter, and uniform distribution of the particles. With the conventional technique, however, since the difference in composition and particle size distribution are so extensive that the sintered bodies having low critical current density could only be obtained.